Glass material and method for manufacturing same

ABSTRACT

Provided is a glass composition that exhibits greater Faraday effect than ever before. A glass composition contains 48% or more of Tb 2 O 3  (exclusive of 48%) in % by mole.

TECHNICAL FIELD

The present invention relates to a glass material suitable for a magneto-optical element making up part of a magnetic device, such as an optical isolator, an optical circulator or a magnetic sensor, and a method for manufacturing the same.

BACKGROUND ART

A glass material containing terbium oxide which is a paramagnetic compound is known to exhibit the Faraday effect which is one of magneto-optical effects. The Faraday effect is an effect of rotating the polarization plane of linearly polarized light passing through a material placed in a magnetic field. This effect is utilized in optical isolators, magnetic field sensors, and so on.

The optical rotation θ (angle of rotation of the polarization plane) due to the Faraday effect is expressed by the following formula where the intensity of a magnetic field is represented by H and the length of a substance through which polarized light passes is represented by L. In the formula, V represents a constant dependent on the type of the substance and is referred to as a Verdet constant. The Verdet constant takes positive values for diamagnetic substances and negative values for paramagnetic substances. The larger the absolute value of the Verdet constant, the larger the absolute value of the optical rotation, resulting in exhibition of greater Faraday effect.

θ=VHL

Conventionally known glass materials exhibiting the Faraday effect include SiO₂—B₂O₃—Al₂O₃—Tb₂O₃-based glass materials (see Patent Literature 1), P₂O₅—B₂O₃—Tb₂O₃-based glass materials (see Patent Literature 2), and P₂O₅—TbF₃—RF₂— (where R represents an alkaline earth metal) based glass materials (see Patent Literature 3).

CITATION LIST Patent Literature [PTL 1]

-   Examined Japanese Patent Application Publication No. S51-46524

[PTL 2]

-   Examined Japanese Patent Application Publication No. S52-32881

[PTL 3]

-   Examined Japanese Patent Application Publication No. S55-42942

SUMMARY OF INVENTION Technical Problem

Although the above glass materials exhibit some degree of Faraday effect, recent increasing size reduction of magnetic devices requires further improvement of the Faraday effect so that even a small member can exhibit a sufficient optical rotation.

In view of the foregoing, an object of the present invention is to provide a glass material exhibiting greater Faraday effect than ever before.

Solution to Problem

A glass material of the present invention contains 48% or more of Tb₂O₃ (exclusive of 48%) in % by mole. Since the glass material of the present invention contains a large amount of Tb₂O₃ as described above, the absolute value of the Verdet constant becomes large. As a result, the glass material exhibits greater Faraday effect than ever before. A glass material containing a large amount of Tb₂O₃ as described above is generally difficult to vitrify. However, a containerless levitation technique as will be described later enables even such a composition difficult to vitrify to be easily vitrified.

The glass material of the present invention preferably has a Tb₂O₃ content of not more than 80% by mole. When the Tb₂O₃ content is in this range, the glass material can be relatively easily vitrified.

The glass material of the present invention preferably further contains, in % by mole, 0 to 50% SiO₂, 0 to 50% B₂O₃, 0 to 50% Al₂O₃, and 0 to 50% P₂O₅. Because SiO₂, B₂O₃, Al₂O₃, and P₂O₅ are components for forming a glass network, incorporation of these components into the glass material enables the glass material to be relatively easily vitrified.

The glass material of the present invention can be used as a magneto-optical element. For example, the glass material of the present invention can be used as a Faraday rotator which is a type of magneto-optical element. The use of the glass material for the above application enables the glass material to be given the effect of the present invention.

A method for manufacturing a glass material of the present invention is a method for manufacturing the glass material described above and includes the step of heating a block of glass raw material to melting while holding the block of glass raw material levitated, thus obtaining a molten glass, and then cooling the molten glass.

Generally, a glass material is produced by melting a row material in a melting container, such as a crucible, and then cooling it (melting method). However, the glass material of the present invention has, as described above, a composition containing a large amount of Tb₂O₃ fundamentally not forming a glass network and is therefore a less vitrifiable material, which may present a problem that the use of the melting method would result in progression of crystallization originating on the contact interface with the melting container.

Even if the glass material has a less vitrifiable composition, vitrification thereof can be achieved by eliminating contact at the interface with the melting container. As such a technique, a containerless levitation technique is known in which a raw material is melted and cooled while held levitated. With the use of this technique, the molten glass is substantially kept out of contact with the melting container, so that crystallization originating on the interface with the melting container can be prevented and the glass material can be thus vitrified.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a glass material exhibiting greater Faraday effect than ever before.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing one embodiment of an apparatus for producing a glass material of the present invention.

DESCRIPTION OF EMBODIMENTS

The glass material of the present invention contains, in % by mole, 48% or more of Tb₂O₃ (exclusive of 48%), preferably 49% or more of Tb₂O₃, and particularly preferably 50% or more of Tb₂O₃. If the Tb₂O₃ content is too small, the absolute value of the Verdet constant becomes small, so that sufficient Faraday effect is less likely to be achieved. On the other hand, if the Tb₂O₃ content is too large, the glass material tends to be difficult to vitrify. Therefore, the Tb₂O₃ content is preferably not more than 80%, more preferably not more than 75%, and particularly preferably not more than 70%.

In relation to Tb, the content of its trivalent oxide has been defined above, but, as for its oxides other than the trivalent oxide, their content when converted to the trivalent oxide is preferably in the above range.

The magnetic moment from which the Verdet constant of Tb is derived is greater in Tb³⁺ than in Tb⁴⁺. A larger percentage of Tb³⁺ in the glass material is more preferable because the Faraday effect becomes greater. Specifically, the percentage of Tb³⁺ in the total content of Tb is, in % by mole, preferably 50% or more, more preferably 60% or more, still more preferably 70% or more, yet more preferably 80% or more, and particularly preferably 90% or more.

The glass material of the present invention may contain, in addition to Tb₂O₃, the following various types of components. In the following description of the contents of components, “%” refers to “% by mole” unless otherwise specified.

SiO₂, B₂O₃, and P₂O₅ are components for forming a glass network and widening the vitrification range. However, these components do not contribute to increase in the Verdet constant. Therefore, if the content of these components is too large, sufficient Faraday effect is less likely to be achieved. Hence, the content of each of SiO₂, B₂O₃, and P₂O₅ is preferably 0 to 50%, more preferably 1 to 45%, and particularly preferably 2 to 40%. Furthermore, the total amount of SiO₂ and B₂O₃ is preferably 0 to 52%, more preferably 15 to 51%, and particularly preferably 20 to 50%. The total amount of B₂O₃ and P₂O₅ is preferably 0 to 52%, more preferably 15 to 51%, and particularly preferably 20 to 50%. The total amount of SiO₂, B₂O₃, and P₂O₅ is preferably 0 to 52%, more preferably 15 to 51%, and particularly preferably 20 to 50%.

Al₂O₃ is a component for forming a glass network as an intermediate oxide and widening the vitrification range. However, Al₂O₃ does not contribute to increase in the Verdet constant. Therefore, if its content is too large, sufficient Faraday effect is less likely to be achieved. Hence, the Al₂O₃ content is preferably 0 to 50%, more preferably 0.1 to 40%, still more preferably 1 to 30%, yet more preferably 1 to 20%, and particularly preferably 1 to 10%.

La₂O₃, Gd₂O₃, Yb₂O₃, and Y₂O₃ have the effect of stabilizing glass, but an excessive large content thereof contrariwise makes the glass material less likely to be vitrified. Therefore, the content of each of La₂O₃, Gd₂O₃, Yb₂O₃, and Y₂O₃ is preferably 10% or less and particularly preferably 5% or less.

Dy₂O₃, Eu₂O₃, and Ce₂O₃ stabilize glass and contributes to increase in the Verdet constant. However, an excessive large content thereof contrariwise makes the glass material less likely to be vitrified. Therefore, the content of each of Dy₂O₃, Eu₂O₃, and Ce₂O₃ is preferably 15% or less and particularly preferably 10% or less. In relation to Dy, Eu, and Ce, the contents of their trivalent oxides have been defined above, but, as for their oxides (for example, CeO₂) other than trivalent oxides, their contents when converted to the trivalent oxides are preferably in the above range.

MgO, CaO, SrO, and BaO have the effect of increasing the stability and chemical durability of glass. However, these components do not contribute to increase in the Verdet constant. Therefore, if the content of them is too large, sufficient Faraday effect is less likely to be achieved. Hence, the content of each of these component is preferably 0 to 10% and particularly preferably 0 to 5%.

Ga₂O₃ has the effect of increasing the glass formation ability and widening the vitrification range. However, an excessive large content thereof makes the resultant glass likely to be devitrified. Furthermore, Ga₂O₃ does not contribute to increase in the Verdet constant. Therefore, if its content is too large, sufficient Faraday effect is less likely to be achieved. Hence, the Ga₂O₃ content is preferably 0 to 6% and particularly preferably 0 to 5%.

Fluorine has the effect of increasing the glass formation ability and widening the vitrification range. However, if its content is too large, fluorine volatilizes during melting, which may vary the glass composition or may have an influence on the stability of glass. Therefore, the fluorine content (in terms of F₂) is preferably 0 to 10%, more preferably 0 to 7%, and still more preferably 0 to 5%.

Sb₂O₃ may be added as a reductant. However, in order to avoid coloration or in consideration of environmental burden, the Sb₂O₃ content is preferably 0.1% or less.

The glass material of the present invention preferably has a light transmission loss as small as possible, particularly when used as a magneto-optical element, such as an isolator. Therefore, the glass material of the present invention has alight transmittance of preferably 50% or more, more preferably 60% or more, and particularly preferably 70% or more at a wavelength of 633 nm.

The glass material of the present invention can be produced, for example, by a containerless levitation technique. FIG. 1 is a schematic cross-sectional view showing an example of an apparatus for manufacturing a glass material by a containerless levitation technique. Hereinafter, a description will be given of a method for manufacturing a glass material of the present invention with reference to FIG. 1.

The apparatus 1 for manufacturing a glass material includes a forming die 10. The forming die 10 also serves as a melting container. The forming die 10 includes a forming surface 10 a and a plurality of gas jet holes 10 b opening on the forming surface 10 a. The gas jet holes 10 b are connected to a gas supply mechanism 11, such as a compressed gas cylinder. Gas is supplied from this gas supply mechanism 11 via the gas jet holes 10 b to the forming surface 10 a. No particular limitation is placed on the type of the gas and the gas may be, for example, air or oxygen or may be nitrogen gas, argon gas, helium gas, carbon monoxide gas, carbon dioxide gas or hydrogen-containing reducing gas.

In manufacturing a glass material using the manufacturing apparatus 1, first, a block 12 of glass raw material is placed on the forming surface 10 a. Examples of the block 12 of glass raw material include a body obtained by forming raw material powders into a single piece by press forming or other processes, a sintered body obtained by forming raw material powders into a single piece by press forming or other processes and then sintering the single piece, and an aggregate of crystals having the same composition as a desired glass composition.

Next, gas is jetted out through the gas jet holes 10 b, thus levitating the block 12 of glass raw material above the forming surface 10 a. In other words, the block 12 of glass raw material is held out of contact with the forming surface 10 a. In this state, the block 12 of glass raw material is irradiated with laser light from a laser light applicator 13. Thus, the block 12 of glass raw material is heated to melting to make it vitrifiable, thereby obtaining a molten glass. Thereafter, the molten glass is cooled, so that a glass material can be obtained. During the step of heating the block 12 of glass raw material to melting and the step of cooling the molten glass and in turn the glass material at least to below the softening point, at least the jetting of gas is preferably continued to restrain the contact of the block 12 of glass raw material, the molten glass, and finally the glass material with the forming surface 10 a. The block 12 of glass raw material may be levitated above the forming surface 10 a using a magnetic force generated by applying a magnetic field. Furthermore, the method for heating the block of glass raw material to melting may be radiation heating besides the method of irradiating it with laser light.

Examples

The present invention will be described below with reference to examples but the present invention is not at all limited by the following examples.

Table 1 shows examples of the present invention and comparative examples.

TABLE 1 Comp. Ex. Ex. 1 2 3 4 5 6 7 1 Glass Composition Tb₂O₃ 50 60 60 50 50 60 80 30 (% by mole) Al₂O₃ 20 5 10 20 5 20 B₂O₃ 30 35 25 20 20 35 17.5 P₂O₅ 5 20 SiO₂ 5 5 10 2.5 50 Verdet Constant 633 nm −0.69 −0.80 −0.79 −0.69 −0.70 −0.79 −1.04 −0.37 (min/Oe · cm) 850 nm −0.34 −0.40 −0.39 −0.34 −0.35 −0.39 −0.52 −0.18

Each sample was prepared in the following manner. First, raw materials formulated to have a glass composition indicated in the table were press-formed and sintered at 1100 to 1400° C. for 12 hours, thus producing a block of glass raw material.

Next, the block of glass raw material was coarsely ground into 0.05 to 0.5 g small pieces. Using the obtained small piece of the block of glass raw material, a glass material (with a diameter of approximately 1 to 8 mm) was produced by a containerless levitation technique using an apparatus conforming to FIG. 1. A 100 W CO₂ laser oscillator was used as a heat source. Furthermore, nitrogen gas was used as a gas for levitating the block of raw material and supplied at a flow rate of 1 to 30 L/min.

The obtained glass material was measured in terms of the Verdet constant using a Kerr effect measuring system (manufactured by JASCO Corporation, Model K-250). Specifically, the obtained glass material was polished to have a thickness of about 1 mm and measured in terms of angle of Faraday rotation at a wavelength of 400 to 850 nm in a magnetic field of 15 kOe and the Verdet constants thereof at wavelengths of 633 nm and 850 nm were calculated. The wavelength sweep speed was 6 nm/min. The results are shown in Table 1.

As is obvious from Table 1, the glass materials of Examples 1 to 7 exhibited Verdet constants of −0.69 to −1.04 at a wavelength of 633 nm and Verdet constants of −0.34 to −0.52 at a wavelength of 850 nm. On the other hand, the Verdet constants of the glass material of Comparative Example 1 were −0.37 at a wavelength of 633 nm and −0.18 at a wavelength of 850 nm, the absolute values of which were small.

INDUSTRIAL APPLICABILITY

The glass material of the present invention is suitable as a magneto-optical element making up part of a magnetic device, such as an optical isolator, an optical circulator or a magnetic sensor.

REFERENCE SIGNS LIST

-   -   1: apparatus for manufacturing a glass material     -   10: forming die     -   10 a: forming surface     -   10 b: gas jet hole     -   11: gas supply mechanism     -   12: block of glass raw material     -   13: laser light applicator 

1: A glass material containing 48% or more of Tb₂O₃ (exclusive of 48%) in % by mole. 2: The glass material according to claim 1, having a Tb₂O₃ content of not more than 80% by mole. 3: The glass material according to claim 1, further containing, in % by mole, 0 to 50% SiO₂, 0 to 50% B₂O₃, 0 to 50% Al₂O₃, and 0 to 50% P₂O₅. 4: The glass material according to claim 1, being used as a magneto-optical element. 5: The glass material according to claim 4, being used as a Faraday rotator. 6: A method for manufacturing the glass material according to claim 1, the method comprising the step of heating a block of glass raw material to melting while holding the block of glass raw material levitated, thus obtaining a molten glass, and then cooling the molten glass. 